(1) Field of the Invention
The present invention relates to a semiconductor device, and more particularly to a method of producing a semiconductor device comprising bipolar transistors and Schottky barrier diodes (SBDs) formed in a semiconductor substrate.
(2) Description of the Prior Art
In a bipolar integrated circuit (IC) such as a transistor-transistor logic (TTL) circuit, in order to prevent a delay of the switching time due to the storage effect of minority carriers, an SBDTTL, wherein the SBD is clamped between the base and the collector of a bipolar transistor, is preferably used. In the production of a highly dense bipolar IC, a washed emitter process, wherein a window for impurity diffusion is also used as an emitter electrode contact window, is utilized for miniaturizing the bipolar transistor. The conventional washed emitter process comprises the steps of forming a window in an insulating layer covering a base region, forming an impurity-containing glass layer in the window, thermally diffusing the impurities of the glass layer into the base region so as to form an emitter region, washing the glass layer with a suitable etchant, and forming an emitter electrode. Improvements in the washed emitter process have been proposed in, for example, Japanese Examined Patent Publication (Kokoku) No. 54-7194 (published Apr. 4, 1979) and No. 55-24703 (published July 1, 1980). According to these publications, after the formation of a window in an insulating layer covering a base region, a polycrystalline silicon layer is formed in the window. Then a silicate glass layer containing highly concentrated impurities is selectively formed on the polycrystalline silicon layer above the window and is heated to diffuse the impurities into the base region through the polycrystalline silicon layer. After the glass layer is washed, an emitter electrode is formed on the polycrystalline silicon layer. However, in an SBD, if a polycrystalline silicon layer is present between a semiconductor layer and a metal electrode, a Schottky barrier cannot be perfectly formed.
Taking the above-mentioned prior art into consideration, a method of producing an SBD clamped transistor is proposed in Japanese Examined Patent Publication No. 55-24697 (published July 1, 1980). In this case, after the formation of all of the contact windows for the collector electrode, the base electrode, the emitter electrode and an electrode of the SBD in an insulating layer formed on a semiconductor layer, a thin silicon dioxide (SiO.sub.2) layer is formed on at least the exposed portions of the semiconductor layer. The thin SiO.sub.2 layer serves to prevent etching of the polycrystalline silicon. The thin SiO.sub.2 layer is removed by etching all of it except the portion in the window for the SBD. A polycrystalline silicon layer is formed on the insulating layer, on the exposed portions of the semiconductor layer in the windows for the electrodes of the transistor, and on the thin SiO.sub.2 layer in the window for the SBD. A silicate glass layer containing impurities is formed on the polycrystalline silicon layer and is selectively etched to remove a portion thereof above the window for the base electrode. The impurities are thermally diffused through the polycrystalline silicon layer by means of a heat treatment so as to form an emitter region. The silicate glass layer is washed and then the polycrystalline silicon layer and the thin SiO.sub.2 layer are selectively etched to expose a portion of the semiconductor layer within the window for the SBD. A metal layer, e.g., an aluminum (Al) layer, is deposited and selectively etched so as to form the electrode of the SBD on the exposed semiconductor layer and the collector electrode, the base electrode and the emitter electrode on the polycrystalline silicon layer within the windows, respectively. However, the proposed method involves the formation of four photoresist layers having different patterns to form the windows and necessitates very careful etching of the thin SiO.sub.2 layer. Thus, the production steps are relatively many and complicated and the production yield can easily vary.